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Notes on tech

Notes on technology, business, enterpreneurship, economy, markets along with interesting general tidbits.


RAID basics

7/24/2005 12:09:00 AM, posted by anand

Here is my summarized version from this article.

RAID stands for Redundant Array of Inexpensive Disks. RAID can be run on any type of hard drive, including SCSI, SATA, and ATA. The number of hard drives required is dependent on the particular RAID configuration chosen

RAID 0:
Using at least two disks, RAID 0 writes data to the two drives in an alternating fashion, referred to as striping. If you had 8 chunks of data, for example, chunk 1, 3, 5, and 7 would be written to the first drive, and chunk 2, 4, 6, and 8 would be written to the second drive, but all in sequential order.

Speed: This process of splitting the data across drives allows for a theoretical performance boost of up to double the speed of a single hard drive, but real world results will generally not be nearly that good. Since all data is not written to each disk, the failure of any one drive in the array generally results in a complete loss of data.

Pros: RAID 0 is good for people who need to access large files quickly, or just demand high performance across the board (i.e. gaming systems).

Cons: No redundancy. Even if one drives blows up, all data is lost.

RAID 1:
RAID 1 is one of the most basic arrays that provides redundancy. Using at least two hard drives, all data is written to both drives in a method referred to as mirroring. Each drive’s contents are identical to each other, so if one drive fails, the system could continue operating on the remaining good drive.

Speed: There is no performance increase as in RAID 0, and in fact there may be a slight decrease compared to a single drive system as the data is processed and written to both drives.

Pros: Cheap and easy to setup. Provides basic data redundancy.

Cons: No speed gain over RAID 0.

RAID 0+1:
Combination of RAID 0 and RAID 1. A minimum of four drives is required to implement RAID 0+1, where all data is written in both a mirrored and striped fashion to the four drives. Chunks 1, 3, 5, and 7 would be written to drives one and three, and chunks 2, 4, 6 and 8 would be written to drives two and four, again in a sequential manner.

Speed: As fast as RAID 0

Pros: Performance boost of RAID 0 and the redundancy of RAID 1.

Cons: Expensive - needs 4 disks.

RAID 5:
Data is striped across all drives in the array, and in addition, parity information is striped as well. This parity information is basically a check on the data being written, so even though all data is not being written to all the drives in the array, the parity information can be used to reconstruct a lost drive in case of failure. Chunks 1 and 2 would be written to drive one and two respectively, with a corresponding parity chunk being written to drive three. Chunks 3 and 4 would then be written to drives one and three respectively, with the corresponding parity chunk being written to drive two. Chunks 5 and 6 would be written to drives two and three, with the corresponding parity chunk being written to drive one. Chunks 7 and 8 take us back to the beginning with the data being written to drives one and two, and the parity chunk being written to drive three.

Pros: Full redundancy. Parity information can be used to reconstruct a lost drive in case of failure.

Cons: Very expensive and complicated.

Other RAID resources:
http://www.acnc.com/raid.html
http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks
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